An alternating magnetic field is a magnetic field which has in its frequency spectrum only frequency components differing from 0, and is thus, in particular, temporally variable.
Conventional optical measuring arrangements and measuring methods measure a magnetic field using the magnetooptic Faraday effect. The Faraday effect is the rotation of the plane of polarization of linearly polarized light as a function of a magnetic field. The rotational angle is proportional to the path integral over the magnetic field along the path covered by the light, with the Verdet's constant as the constant of proportionality. The Verdet's constant is generally a function of material, temperature and wavelength. A Faraday sensor device comprising an optically transparent material such as, for example, glass is arranged in the magnetic field for the purpose of measuring the latter. The magnetic field causes a rotation of the plane of polarization of linearly polarized light, transmitted by the Faraday sensor device, by a rotational angle which can be evaluated for a measuring signal. Such magnetooptic measuring methods and measuring arrangements are may to be used to measure electric currents. The Faraday sensor arrangement is arranged for this purpose in the vicinity of an electric conductor, and detects the magnetic field generated by a current in the electric conductor. In general, the Faraday sensor device surrounds the electric conductor, so that the measuring light runs round the electric conductor in a closed path. The absolute value of the rotational angle is in the current to be measured. The Faraday sensor device can be constructed as a solid glass ring around the electric conductor, or else can surround the electric conductor in the form of a measuring coil comprising a light-conducting fiber (fiber coil) with at least one turn.
The advantages of magnetooptic measuring arrangements and measuring methods by comparison with conventional inductive current transformers are the electrical isolation and the insensitivity with respect to electromagnetic interference. However, temperature influences, and in particular influences of mechanical bending and vibration in the sensor device and the optical transmission links, in particular optical fibers for transmitting the measuring light, present problems.
International Patent Application No. WO 95/10045 describes a conventional device where two lineraly polarized light signals are transmitted in opposite directions through a Faraday sensor device. The Faraday sensor device surrounds an electric conductor, and has a circular birefringence which is negligible by comparison with the Faraday effect. After traversing the sensor device, each of the two light signals is decomposed by a polarizing beam splitter into two mutually perpendicular linearly polarized component light signals. For each light signal, an intensity-normalized signal is formed, which corresponds to the quotient of a difference and the sum of the two associated component light signals. A signal processor derives from the two intensity-normalized signals a measuring signal for an electric current in the electric conductor, the signal being virtually independent both of the temperature and of vibrations in the sensor device.
In three other conventional devices, two light signals traverse an optical series circuit comprising a first optical fiber, a first polarizer, a Faraday sensor device, a second polarizer and a second optical fiber, doing so in mutually opposite directions of circulation. After traversing the optical series circuits, the two light signals are transformed into an electric intensity signal in each case by appropriate photoelectric transducers.
In one conventional measuring system described in U.S. Pat. No. 4,916,387, a solid glass ring which surrounds the electric conductor is provided as Faraday sensor device. The axes of polarization of the two polarizers are rotated by an angle of 45.degree. relative to one another. In order to compensate for undesired changes in intensity in the optical supply fibers, it is assumed in measuring system that the undesired changes in intensity (noise) and the changes in intensity owing to the Faraday effect are superimposed on one another additively with different signs in the two electric intensity signals, and can thus be separated from one another.
In a second conventional measuring system, described in
Journal of Lightwave Technology, Vol.12, No.10, Oct. 1994, pages 1882 to 1890, a fiber coil comprising a single-mode fiber with low birefringence is provided as Faraday sensor device. The axes of polarization of the two polarizers enclose with one another a polarizer angle differing from 0.degree. , which is preferably 45.degree.. Light from a light source is split into two light signals, and these two light signals are respectively launched into the Faraday fiber coil via an optical coupler and an assigned transmitting optical fiber at opposite ends. A measuring signal which corresponds to the quotient (I1-I2)/(I1+I2) of the difference and the sum of the two intensity signals is derived from two electric intensity signals I1 and I2 which correspond to the light intensities of the two light signals after traversal of the series circuit. The attenuations of the two optical fibers can essentially be compensated thereby. The light intensities of the two light signals upon launching into the series circuit must, however, be set to be exactly equal.
In a third conventional magnetooptic measuring system, described in H. Sohlstrom et al, "Transmission loss compensation for Faraday effect fiber optic sensors", Conference Eurosensors VIII, Toulouse, 25.-28.9.1994 an optical series circuit comprising a first arrangement, a second polarizer and a second multimode fiber are connected between two infrared light-emitting diodes. The two light-emitting diodes are operated alternately as light source and as photodetector. Thus, at any given instant only one of the two oppositely directed light signals is traversing the series circuit. The switching clock frequency is therefore selected to be as high as possible.
In the three conventional measuring systems described above a measuring signal (intensity-normalized measuring signal) is derived which is independent of intensity losses in the common light path for the two oppositely directed light signals. It is assumed in this case that these intensity losses are equal for both light signals, and that the common light path is therefore reciprocal.
A magnetooptic current transformer is described in "Optical Combined Current & Voltage H.V. Sensors, GEC Alsthom, T&D", Here, a light signal linearly polarized in a polarizer traverses a Faraday glass ring and thereafter is split by a polarizing beam splitter into two component light signals linearly polarized perpendicular to one another (two-channel polarization evaluation). The two component light signals are fed, in each case, via one optical fiber to an associated photodiode which converts the corresponding component light signal into an electrical intensity signal S1 or S2 which is proportional to the light intensity of the associated component light signal.
Owing to different attenuation in the two optical fibers, the two constants of proportionality can now be different from one another. A special control is provided for the purpose of compensating these differences in sensitivity. A controllable first amplifier connected downstream of the first photodiode amplifies the intensity signal S1 by an associated gain factor K1, and a second amplifier connected downstream of the second photodiode amplifies the second intensity signal S2 by a second gain factor K2. Direct signal components (DC values) of the two intensity signals S1 and S2 are now determined, and the difference between the two direct signal components is controlled as control variable to zero by controlling the gain factor K1 of the first amplifier. A measuring signal which corresponds to the quotient [(K1.multidot.S1)-(K2.multidot.S2)]/[(K1.multidot.S1)+(K2.multidot.S2)] of the difference and the sum of the output signals of the amplifiers is now formed from the two intensity signals K1.multidot.S1 and K2.multidot.S2, generally amplified at different strengths, at the outputs of the two amplifiers.